Generally speaking, automatic transmission of the so-called "Simpson type" has two single planetary gear units which share sun gears and in which the carrier of the first planetary gear unit and the ring gear of the second planetary gear unit are integrally connected to each other. Thus, the forward 3rd speed is established by selectively inputting to the ring gear or sun gear of the first planetary gear unit through a clutch, by stopping certain rotary components of the planetary gear units by brakes or one-way clutches, and by outputting from the ring gear of the second planetary gear unit connected integrally to the carrier of the first planetary gear unit.
A four-speed prior art automatic transmission is based upon the aforementioned three-speed automatic transmission mechanism composed of the two planetary gear units and establishes a forward 4th gear stage by addition of an over-drive (O/D) mechanism or an underdrive (U/D) mechanism composed of a single planetary gear unit.
In the automatic transmission having the O/D mechanism, moreover, the output member, i.e., the ring gear of this O/D mechanism, is axially connected to the input member of the three-speed automatic transmission mechanism.
In the automatic transmission shown in FIG. 6, for example, an O/D mechanism is attached to a three-speed automatic transmission mechanism 10.sub.1 (as shown at a lower half with respect to line L of FIG. 6) to provide a four-speed automatic transmission mechanism (as shown at an upper half from the line L of FIG. 6). This example is equipped with an O/D mechanism clutch C0, through which rotation of the input shaft is transmitted to the ring gear R2 of the dual planetary gear units.
In view of recent trends toward the FF type automobile and a higher output power, the automatic transmission is desired to have a smaller size and a larger capacity of transmission torque. However, these objectives cannot be satisfied by the aforementioned four-speed automatic transmission which is composed of the three planetary gear, including an O/D mechanism or the like, as has been described in connection with the prior art.
Specifically, in the example shown in FIG. 6, the O/D mechanism (i.e., including the clutch C0) is attached to the input shaft of the three-speed transmission mechanism so that the axial size of the automatic transmission is increased. Since, moreover, the ring gear R2 or the input member of the O/D mechanism is connected by an extension radially outward of the clutch C1, the radial size of the automatic transmission is also increased.
According to the simplest structure, moreover, the r.p.m. sensor of the input shaft is attached to the transmission casing 17a to measure the r.p.m. of the drum 130 of the clutch C0. With this structure, however, the r.p.m. of the input shaft cannot be measured from the clutch C0 drum 130 which is not rotated at all times.